The enzyme ornithine decarboxylase is stimulated by epidermal growth factor. It is inhibited by phosphorylation of tyrosine shown in experiments with slime mold enzyme. We hope that the isolation of this enzyme from rat liver using published methods will yield in the mammalian enzyme the same information on inhibitory phosphorylation on tyrosine. We want to see if its phosphorylation increased on RSV infection or by in vitro phosphorylation with pp60src. There are indications that in ornithine decarboxylase a modulation of activity may be due to an alteration of phosphorylation and phosphohydrolase. In early experiments temperature dependence of RSV transformation has been connected with the absence of tyrosine-phosphorylation in pp60src enzyme at non-permissive temperature as against its presence at permissive temperature. This might indicate a stimulation of hydrolysis of this enzyme-bound tyrosine phosphate at the non-permissive higher temperature. The role of tyrosine phosphorylation in enzyme action has not been definitely decided. It may have either a modulatory effect on the enzyme or indicate an intermediary binding of phosphate in the activity of the enzyme as phosphotransferase. Work on stimulation of phosphorylation of the ribosomal protein s6 by infection with RSV showed a multiple phosphorylation of serine. We are now trying to purify the kinase responsible for this effect which indicates that not only phosphorylation of tyrosine but also of serine may be stimulated by RSV infection. On the other hand studies on general phosphorylation of ribosomal proteins showed some phosphorylation involving tyrosine. Another protein phosphorylated on RSV infection, in this case on tyrosine, is the much studied 34K protein. Phosphorylation causes an increase from 34K to 38K and we want to use this difference to study intracellular distribution. A recent report indicates a frequent sulfurylation of tyrosine in normal tissue proteins and also in a tumor. In view of an earlier interest in the mechanism of sulfate transfer in mammalian tissues it should be easy to design a method to follow sulfurylation in tissue extracts and also see if theere is some specificity of tyrosine sulfurylation in tumor tissue.